// $Id: README 80826 2008-03-04 14:51:23Z wotte $

This directory contains the HTTP Tunneling, Bidirectional, Protocol
implementation. This is a new streaming abstraction layered over an
HTTP document request/reply mechanism. It is designed to allow clients
that are inside a of a corporate firewall to communicate with servers
that are outside, hence HTTP Tunneling. Also, once a connection is
established, the outside peer is able to send asynchronous messages to
the inside peer, hence Bidirectional.

HTBP provides Acceptor, Connector, and Stream classes that follow the
interface defined for the ACE_Acceptor and ACE_Connector
templates. This means that HTBP can be used right away with
applications designed to use these templates.

Bidirectionality is achieved in the context of the proxy's restriction
by using two channels between the peers. One channel is defined for
data flow from the inside to the outside, data flow from the outside
in occurs on the other channel. In-to-out data is passed in the form
of a PUT command with the data payload marshalled into an
"application/gzip" buffer. On this channel, the outside peer always
responds with a simple document which serves as an ack. On the
out-to-in channel, the inside client must send a token request in the
form of a GET command. This request goes unfulfilled until the outside
peer has data to send, which it does by supplying an HTML document
that again encapsulates the data so that it may pass through the proxy
uncorrupted.

The connections from the inside peer to the proxy, or from the proxy
to the outside peer may be closed by the proxy at any time. The inside
peer will automatically reconnect as needed when this happens. Since
the outside peer cannot actively connect to the proxy, it will queue
outbound messages as long as it can until a new out-to-in channel is
made available, at which time it will flush its queue. The sense of
channels may change over time, as the proxy may choose any local
connection to the server to send any request (GET or POST).

The outside peer is identified using an ordinary INET addr, however
the inside peer uses a simple, transient unique ID to identify
itself. Inside peers will never have any type of persistent identity.
The unique ID used to identify the inside peer is usually a UUID value
as composed by ACE_UUID_Generator. However, a domain based unique ID
may also be obtainedusing HTBP::ID_Requestor::get_HTID(). If no domain
based ID generator is configured, get_HTID() will return a UUID value.

As there are a variety of HTTP proxies available, HTBP uses a
pluggable filter class that defines the particular characteristics of
a proxy and is responsible for marshalling and unmarshalling binary
data. As of now there is a single filter available that works with a
defaulted Squid proxy and may also be used as a null filter, directly
connecting the inside and outside peers. This mode is useful for
testing.

CONFIGURING HTBP
This is done through the ACE_Configuration framework. On windows
platforms, the Windows Registry may be used, whereas on non-windows, a
flat file is used to configure. Configuration data may also be
persisted in a memory mapped file.

The configuration map contains a single section, HTBP, that contains
all the configurable parameters in name=value form. The following is
an example of a configuration file:

[htbp]
proxy_host=<hostname>               This is the hostname of the http
                                    proxy through which all requests
                                    will be sent.
proxy_port=<port>                   This is the proxy's port.
htid_url=<url>                      If a domain based unique id is
                                    required, this is the URL of the
                                    ID generator.
htid_via_proxy=<1|0>                If the htid_url must be reached
                                    via the proxy, set this to 1.
                                    Default is 0, meaning the ID
                                    generator is directly accessible.

COMPANION DIRECTORIES:
$ACE_ROOT/tests/HTBP.               These are the test drivers, which
                                    also serve as example code.
$TAO_ROOT/orbsvcs/orbsvcs/HTIOP     This is a TAO pluggable protocol
                                    based on HTBP.
$TAO_ROOT/orbsvcs/tests/HTIOP       The tests for HTIOP.


BACKGROUND INFORMATION

HT Addresses

The class HT_Addr is a subclass of ACE_INET_Addr class. The interface
for the HT_Addr is a common interface to be used with the inside and
outside peers. The inside peer is identified by a HTID while the
outside peer is identified with an ip address.  Constructors are
provided to initialize the inside and outside peers in addition to the
default and copy constructors.  addr_to_string and string_to_addr
methods from the base class are overridden to help convert the HT_Addr
to a string and vice versa. Finally, the class provides accessor
methods for the default local address and the default proxy addresses.

The local address is the address of the inside peer and is obtained
using the singleton HTID_Requestor class. The HTID_Requestor class
sends a request to the web server that is running at the htid_url to
get the HTID unique to each inside peer.

The proxy address is of ACE_INET_Addr type as it is no different to a
regular server. It is obtained using the singleton HT_Environment
class. The HT_Environment class helps read the HT configuration file
and provides acccessors to the proxy address, port and the htid url.

The code below illustrates the initialization of a local or inside,
remote or outside and the proxy addresses using the classes
aforementioned.

 HT_Addr local(HTID_REQUESTOR::instance()->get_HTID());

    char hostname [1000];
    if (ACE_OS::hostname (hostname,
		      1000) == -1)
    {
      ACE_DEBUG ((LM_DEBUG, "Could not get the host name\n"));
      return -1;
    }

    HT_Addr remote (8088, hostname);

    char proxy_address [1000];
    HT_ENVIRONMENT::instance ()->get_proxy_address (proxy_address);

    unsigned int proxy_port;
    HT_ENVIRONMENT::instance ()->get_proxy_port (proxy_port);

    ACE_INET_Addr proxy(port, proxy_address);


HT Streams

The class HT_Stream is a sibling of the ACE_SOCK_IO class. It is used
to send and receive messages between two peers identified by their HT
addresses.  It is made a sibling of the ACE_SOCK_IO class rather than
a decendant. This is due to the requirement in the HTBP protocol to
wrap all messages with an HTTP request or reply wrapper, and to send
application data in only one direction on one stream.

HT Sessions

A session is an entity that combines two HT_Streams that connect
directly to a proxy to manage communication with a remote peer. The
session may persist longer than either stream, assuming that the proxy
is libel to close a connection at any time. This means that the
session needs to be able to reconnect to the remote peer. This also
means that the session needs to be aware of its location.  If it is
outside the proxy and looses a stream, nothing can really be done. If
it is inside, then the next time a stream is required, then it must
reconnect before returning the stream. The session does not queue
outbound messages. It will be the responsibility of the application or
a higher level protocol wrapper.

Each session is identified by a special type,
HT_Session_Id_t. HT_Session_Id_t is a class with three members, the
local address, the peer address and an id of type ACE_UINT32. A
session map, ....

Besides the default constructor and copy constructors, two other
constructors are provided to initialize a session and are shown below.


  /// Constructor (sets the underlying session id with <sid>).
  HT_Session (const HT_Addr& peer,
	    const HT_Addr& local = HT_Addr::default_local(),
	    ACE_UINT32 sid = 0,
              ACE_INET_Addr *proxy = 0,
              int take_proxy = 0);

  HT_Session (const HT_Session_Id_t &id,
              ACE_INET_Addr *proxy = 0,
              int take_proxy = 0);



If a session id (sid) is not provided by the user, it is generated
using the static method HT_Session::next_session_id().

The following code illustrates the usage of HT_Stream and HT_Session
classes.

HT_Filters
<TBD>
